Habitat-Driven Variation in Sexual Dimorphism of Amphipods

Sexual dimorphism in morphological traits is widespread across animals and can result from differing life-history strategies, sex-specific competitive pressures, and ecological interactions, which may be influenced by habitat structure and complexity. For epifaunal organisms, e.g., amphipods that inhabit structurally diverse benthic habitats, the structure of the habitat plays a key role in mediating access to food, mate encounters, and refuge provision. Here, we explored patterns of variation in body size and gnathopod 2 ratio (gnathopod 2 length/body length) among amphipod species in different marine habitats. We focus on two amphipod species, Ampithoe ramondi and Caprella acanthifera, across four benthic habitat types: rhodolith beds, macroalgae- dominated reefs, seagrass meadows, and black coral forests. A. ramondi was present in all habitats except black coral forests, and males were significantly larger than females only in macroalgae-dominated reefs. Males also exhibited higher gnathopod ratios than females, increasing from macroalgae-dominated reefs to seagrass meadows and rhodolith beds. C. acanthifera was found in macroalgae-dominated reeds and black coral forests, where males were larger than females on average, but no significant habitat alterations were detected. Neither A. ramondi nor C. acanthifera was found in all four habitats. These results suggest that patterns of sexual dimorphism across coastal habitats are species-specific, with sexual selection operating more subtly in some taxa (e.g., C. acanthifera) than others, likely shaped by both habitat-specific ecological pressures and differences in life-history strategies. Expanding such analyses to more taxa and with balanced sampling across habitats and environmental gradients will offer deeper insight into how natural and sexual selection interact and inform how these dynamics may shift under changing climate regimes.